Newton-Raphson Iteration sequence

Hey can someone please help me with the Newton  Raphson method.
Let f : [a, b] → IR be twice diﬀerentiable, that is f(x) exists for all x ∈ [a, b] and there are positive numbers m and M

|f(x)| ≥ m and 0 < |f(x)| ≤ M for all x ∈ [a, b].

We know that f and f cant change sign on [a,b] and we suppose that f(a)f(b) < 0 (that is, one of f(a) or f(b) is negative). Thus, there is a unique r ∈ [a, b] with f(r) = 0 and f is one-to-one on [a, b]
We define a sequence {an} by a1 = a if ff< 0 otherwise a1 = b and

for n 2

I have already proved that

f(x) = f( ) + ( )(x − ) + (1/2) (x −

HOW DO I SHOW thatr < < ≤ b for all n. Thus, is a decreasing sequence. Let its limit be s